Skeletal muscle provides the contractile force necessary for not only locomotion but also maintenance of posture and respiration. Diseases that weaken or damage skeletal muscle therefore decrease not only quality of life but are, in cases such as Duchenne's muscular dystrophy, fatal. Differentiated muscle cells are terminally postmitotic and cannot generate new myonuclei to replace those lost to damage or disease; a population of adult stem cells termed satellite cells serves to supply new committed muscle cells for muscle hypertrophy, repair, and regeneration. Satellite cells are distributed throughout the muscle and it is well- established that if they are within an area of muscle that is damaged they will activate from their usual quiescent state and proliferate to form a pool of replacement myoblasts, which will differentiate into new muscle fibers or fuse with damaged ones to rapidly and effectively repair the muscle damage. An area of increasing interest is the potential for satellite cells distal to an area of damage to be recruited and relocated through cell migration; this would enhance the efficiency of satellite cell-mediated muscle regeneration. Therapeutic engraftments studies in which satellite cells are injected into dystrophic muscle have met with little success, and one reason has been a failure of injected cells to spread and efficiently engraft. The experiments we propose will explore the ability of factors released by damaged muscle either to promote general motility of satellite cells or to stimulate homing of satellite cels to a specific area; these studies are necessary to define the number and nature of the factors that would require further in-depth study in a later proposal. An understanding of the capacity of satellite cells to move and relocate in vivo, and the role those two activities play in muscle regeneration, would therefore add not only to basic understanding of muscle regeneration but also impact clinical therapies for DMD and other muscle diseases. In the course of these experiments, we are also working towards improved methods for automated tracking of cells in culture, in collaboration with Dr. Kanappan Palaniappan. We will produce an extremely large dataset of tracked cells, which is necessary to develop algorithms capable of replicating tracking choices made by a human researcher. Given the dramatic increase in both video data in multiple contexts and interest in cell motility in this and other systems, software tools that can accurately detect and track changes in position in a complex, high-background environment will be useful to many groups working in stem cells, development, and cell biology.